Multilayer devices are becoming smaller and more dense with the evolution of new technology. The manufacturers of these types of products are therefore constantly challenged to improve their products. One way to improve would be to identify and eliminate defects. Whereas significant improvements are being made to eliminate systematic defects by reducing process variability. Process improvements alone are not sufficient to eliminate all the random defects which effect both yield and reliability. Historically, screening techniques have been employed to improve product failure rates to acceptable levels by culling out many of these random defects. Another way would be through innovation.
Multilayer ceramic (MLC) structures are primarily used in the production of electronic substrates and devices. However, the MLC products also find use in other areas of technology.
The MLCs can have various layering configurations and other features. For example, an MLC circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched in between ceramic layers which act as a dielectric medium. One or more of the ceramic layers have tiny holes which are also called via holes. Prior to lamination, the via holes are filled with electrically conductive metal paste and sintered to form vias which provide the electrical connection between the layers.
In addition, the MLC substrates may have termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, etc.
Generally, conventional ceramic structures are formed from ceramic green sheets which are prepared from a slurry of ceramic particulate, thermoplastic polymer binders, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After punching, metal paste screening, stacking and laminating, the green sheets are fired at temperatures sufficient to burn-off or remove the unwanted polymeric binder resin and sinter the ceramic particulate together into a densified ceramic substrate. Thus the present invention is directed to the screening, stacking and lamination steps of this process.
In the MLC packaging industry it is very common to use green sheets of various thicknesses. The thicknesses can typically vary from 6 mils to 30 mils and in general the art of punching and metallizing these layers are well known. Presently, green sheet thicknesses below 6 mils are very scarcely used. Handling, screening and stacking of green sheets thinner than 6 mils in general poses tremendous challenge. In fact the use of one to two mils green sheets which are punched and screened using traditional MLC technology are currently being invented.
Also, in the MLC packaging industry it is very common to use capacitor layers. The capacitance necessary in a package depends on the design and such capacitance is obtained by choosing proper dielectric layer thickness and metal area within a layer. The industry is striving for higher capacitance and since the metal area is maxing out for a given substrate size it is necessary to use thinner dielectric layers between electrodes to obtain the required capacitance. For example, as a rule of thumb one could double the capacitance for a given dielectric system and electrode metal area by decreasing the dielectric layer thickness by half. Additionally the number of layers needed for capacitance in a package as well is reduced by about 50 percent. Reduction in number of layers is a positive thing in terms of the cost of making the substrate.
The thickness of the thin sheets could be anywhere from 0.5 mil and above. Screening and stacking of thin sheets is very difficult with current technology as they tend to shrink a lot and distort. Furthermore, each layer may need several passes of screening. For example, in one pass one would fill the vias using metal mask, dry the metal paste (and ceramic green sheet) at elevated temperatures, such as, at 80.degree. C. With a second pass one would screen the pattern using mesh screening and dry the paste. There may even be a third pass screening using mesh on the opposite side of the same green sheet and drying the paste. During every pass one could build an uneven paste height on the green sheet which would impact the next pass screening due to, for example, the mask not laying flat on the green sheet, etc. This could lead to paste bleed out in subsequent screening and would result in a non-manufacturable structure and process. Furthermore, the repeated screening and drying process could also make the green sheets curl and wrinkle and hence further screening or stacking of the green sheets would be very difficult.
U.S. Pat. No. 5,024,883 (SinghDeo) teaches building an MLC substrate using a serial process, and uses a thin blanket metal for conductor formation. Also taught is an etching process to etch out the area where conductor is not needed. He also teaches the hot pressing of components.
U.S. Pat. No. 5,480,503 (Casey) teaches releasably-supporting the thin green sheets on a temporary carrier support having an ablatable release layer over a patterned conductive layer, and filling the vias with conductive metal paste, whereby the thin green sheets are supported against warpage and distortion. The supported green sheets are formed as single layers, pairs and stacks thereof, and separated from temporary support for use. The suggested temporary support is a glass plate. The metallization technique is CVD type plating and in the process has to use non-ablatable and ablatable films on the green sheet.
U.S. Pat. No. 5,502,013 (James) addresses a method to form a smooth and flat sintered ceramic surface. Here a greenware precursor is added on a greenware base surface and in sintering a smooth surface is achieved.
U.S. Pat. No. 5,534,092 (Ogawa) teaches the use of two step lamination of stacked green sheets in order to suppress deviation of conductor films and to effectively remove voids by using a modified lamination punch.